We study genes of the rabbit immune system by techniques of molecular biology and immunology. The rabbit has a limited number of VH genes that rearrange. As in the chicken, the 3?-most VH1 gene is rearranged in most rabbit B lymphocytes. Rabbit appendix and chicken bursa of Fabricius are primary lymphoid organs where the B cell antibody repertoire develops in germinal centers mainly by a gene conversion-like process. By six weeks after birth, diversification of rearranged VH genes occurs at least in part, by gene conversion-like events in the appendix suggesting that this organ is a homologue of the avian bursa of Fabricius. Rad51 contributes to repair of double strand breaks in DNA during somatic and meiotic recombination. The gene was first identified in lower eukaryotes and later in vertebrates including chicken, as encoding an E. coli RecA-like protein. We cloned and sequenced RAD51 cDNA from the rabbit. Because the chicken bursa was shown to express high levels of RAD51 message, we investigated the expression of RAD51 in the rabbit appendix and other tissues. Using a quantitative PCR mimic assay and conventional Northern analyses, we found high RAD51 expression in young rabbit appendix comparable to levels in testis where there is an abundance of meiotic recombination. RAD51 levels were three times higher in appendix B lymphocytes compared to T lymphocytes and were lower in adult appendix, as well as in spleen and Peyer?s patches of young rabbits. We measured the levels of message in several appendix cell subpopulations obtained by fluorescence activated cell sorting (FACS) and found that subpopulations of B lymphocytes corresponding to different stages of B-cell development as well as B cells undergoing isotype switch did not have significantly different mRNA levels. We conclude that although Rad51 may have roles in gene conversion and isotype switching in young rabbit appendix, they are played against a background of high steady state RAD51 mRNA levels in developing B cells (Schiaffella, et al., 1998). We investigated the molecular basis for the appearance of VHa2 allotype-bearing B cells in mutant Alicia rabbits. The mutation arose in an a2 rabbit; mutants exhibit altered expression of VH genes because of a small deletion encompassing VH1a2, the 3? most gene in the VH locus. The VH1 gene is the major source of VHa allotype because this gene is preferentially rearranged in normal rabbits. In young homozygous ali/ali animals, the levels of a2 molecules found in the serum increase with age. In adult ali/ali rabbits, 20 to 50% of serum immunoglobulins and B cells bear a2 allotypic determinants. Previous studies suggested that positive selection results in expansion of a2 allotype-bearing B cells in the appendix of young mutant ali/ali rabbits (reviewed in Pospisil and Mage 1998a). We separated appendix cells from a 6- wk-old Alicia rabbit by FACS based on the expression of surface IgM and a2 allotype. The VDJ portion of the expressed immunoglobulin mRNA was amplified from the IgM+ a2+ and IgM+ a2- populations by RT-PCR. The cDNAs from both populations were cloned and sequenced. Analysis of these sequences suggested that in a2+ B cells, the first D proximal functional gene in Alicia rabbits, VH4a2, rearranged and was altered further by a gene conversion-like mechanism. Upstream VH genes were identified as potential gene sequence donors; VH9 was found to be the most frequently used gene donor. Among the a2- B cells y33 was the most frequently rearranged gene (Sehgal et al., 1998).